NAME

LIBRARY

SYNOPSIS

DESCRIPTION

The ptrace() system call provides tracing and debugging facilities. It
allows one process (the tracing process) to control another (the traced
process). The tracing process must first attach to the traced process,
and then issue a series of ptrace() system calls to control the execution
of the process, as well as access process memory and register state. For
the duration of the tracing session, the traced process will be
“re-parented”, with its parent process ID (and resulting behavior)
changed to the tracing process. It is permissible for a tracing process
to attach to more than one other process at a time. When the tracing
process has completed its work, it must detach the traced process; if a
tracing process exits without first detaching all processes it has
attached, those processes will be killed.
Most of the time, the traced process runs normally, but when it receives
a signal (see sigaction(2)), it stops. The tracing process is expected
to notice this via wait(2) or the delivery of a SIGCHLD signal, examine
the state of the stopped process, and cause it to terminate or continue
as appropriate. The signal may be a normal process signal, generated as
a result of traced process behavior, or use of the kill(2) system call;
alternatively, it may be generated by the tracing facility as a result of
attaching, system calls, or stepping by the tracing process. The tracing
process may choose to intercept the signal, using it to observe process
behavior (such as SIGTRAP), or forward the signal to the process if
appropriate. The ptrace() system call is the mechanism by which all this
happens.
The request argument specifies what operation is being performed; the
meaning of the rest of the arguments depends on the operation, but except
for one special case noted below, all ptrace() calls are made by the
tracing process, and the pid argument specifies the process ID of the
traced process or a corresponding thread ID. The request argument can
be:
PT_TRACE_ME This request is the only one used by the traced process; it
declares that the process expects to be traced by its
parent. All the other arguments are ignored. (If the
parent process does not expect to trace the child, it will
probably be rather confused by the results; once the traced
process stops, it cannot be made to continue except via
ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as
execv(3)), it will stop before executing the first
instruction of the new image. Also, any setuid or setgid
bits on the executable being executed will be ignored.
PT_READ_I, PT_READ_D
These requests read a single int of data from the traced
process's address space. Traditionally, ptrace() has
allowed for machines with distinct address spaces for
instruction and data, which is why there are two requests:
conceptually, PT_READ_I reads from the instruction space
and PT_READ_D reads from the data space. In the current
FreeBSD implementation, these two requests are completely
identical. The addr argument specifies the address (in the
traced process's virtual address space) at which the read
is to be done. This address does not have to meet any
alignment constraints. The value read is returned as the
return value from ptrace().
PT_WRITE_I, PT_WRITE_D
These requests parallel PT_READ_I and PT_READ_D, except
that they write rather than read. The data argument
supplies the value to be written.
PT_IO This request allows reading and writing arbitrary amounts
of data in the traced process's address space. The addr
argument specifies a pointer to a structptrace_io_desc,
which is defined as follows:
struct ptrace_io_desc {
int piod_op; /* I/O operation */
void *piod_offs; /* child offset */
void *piod_addr; /* parent offset */
size_t piod_len; /* request length */
};
/*
* Operations in piod_op.
*/
#define PIOD_READ_D 1 /* Read from D space */
#define PIOD_WRITE_D 2 /* Write to D space */
#define PIOD_READ_I 3 /* Read from I space */
#define PIOD_WRITE_I 4 /* Write to I space */
The data argument is ignored. The actual number of bytes
read or written is stored in piod_len upon return.
PT_CONTINUE The traced process continues execution. The addr argument
is an address specifying the place where execution is to be
resumed (a new value for the program counter), or
(caddr_t)1 to indicate that execution is to pick up where
it left off. The data argument provides a signal number to
be delivered to the traced process as it resumes execution,
or 0 if no signal is to be sent.
PT_STEP The traced process is single stepped one instruction. The
addr argument should be passed (caddr_t)1. The data
argument provides a signal number to be delivered to the
traced process as it resumes execution, or 0 if no signal
is to be sent.
PT_KILL The traced process terminates, as if PT_CONTINUE had been
used with SIGKILL given as the signal to be delivered.
PT_ATTACH This request allows a process to gain control of an
otherwise unrelated process and begin tracing it. It does
not need any cooperation from the to-be-traced process. In
this case, pid specifies the process ID of the to-be-traced
process, and the other two arguments are ignored. This
request requires that the target process must have the same
real UID as the tracing process, and that it must not be
executing a setuid or setgid executable. (If the tracing
process is running as root, these restrictions do not
apply.) The tracing process will see the newly-traced
process stop and may then control it as if it had been
traced all along.
PT_DETACH This request is like PT_CONTINUE, except that it does not
allow specifying an alternate place to continue execution,
and after it succeeds, the traced process is no longer
traced and continues execution normally.
PT_GETREGS This request reads the traced process's machine registers
into the “structreg” (defined in <machine/reg.h>) pointed
to by addr.
PT_SETREGS This request is the converse of PT_GETREGS; it loads the
traced process's machine registers from the “structreg”
(defined in <machine/reg.h>) pointed to by addr.
PT_GETFPREGS This request reads the traced process's floating-point
registers into the “structfpreg” (defined in
<machine/reg.h>) pointed to by addr.
PT_SETFPREGS This request is the converse of PT_GETFPREGS; it loads the
traced process's floating-point registers from the “structfpreg” (defined in <machine/reg.h>) pointed to by addr.
PT_GETDBREGS This request reads the traced process's debug registers
into the “structdbreg” (defined in <machine/reg.h>)
pointed to by addr.
PT_SETDBREGS This request is the converse of PT_GETDBREGS; it loads the
traced process's debug registers from the “structdbreg”
(defined in <machine/reg.h>) pointed to by addr.
PT_LWPINFO This request can be used to obtain information about the
kernel thread, also known as light-weight process, that
caused the traced process to stop. The addr argument
specifies a pointer to a structptrace_lwpinfo, which is
defined as follows:
struct ptrace_lwpinfo {
lwpid_t pl_lwpid;
int pl_event;
int pl_flags;
sigset_t pl_sigmask;
sigset_t pl_siglist;
siginfo_t pl_siginfo;
};
The data argument is to be set to the size of the structure
known to the caller. This allows the structure to grow
without affecting older programs.
The fields in the structptrace_lwpinfo have the following
meaning:
pl_lwpid
LWP id of the thread
pl_event
Event that caused the stop. Currently defined
events are
PL_EVENT_NONE
No reason given
PL_EVENT_SIGNAL
Thread stopped due to the pending signal
pl_flags
Flags that specify additional details about
observed stop. Currently defined flags are:
PL_FLAG_SCE
The thread stopped due to system call
entry, right after the kernel is entered.
The debugger may examine syscall arguments
that are stored in memory and registers
according to the ABI of the current
process, and modify them, if needed.
PL_FLAG_SCX
The thread is stopped immediately before
syscall is returning to the usermode. The
debugger may examine system call return
values in the ABI-defined registers and/or
memory.
PL_FLAG_EXEC
When PL_FLAG_SCX is set, this flag may be
additionally specified to inform that the
program being executed by debuggee process
has been changed by succesful execution of
a system call from the execve(2) family.
PL_FLAG_SI
Indicates that pl_siginfo member of structptrace_lwpinfo contains valid information.
pl_sigmask
The current signal mask of the LWP
pl_siglist
The current pending set of signals for the LWP.
Note that signals that are delivered to the process
would not appear on an LWP siglist until the thread
is selected for delivery.
pl_siginfo
The siginfo that accompanies the signal pending.
Only valid for PL_EVENT_SIGNAL kind of stop, when
pl_flags has PL_FLAG_SI set.
PT_GETNUMLWPS
This request returns the number of kernel threads
associated with the traced process.
PT_GETLWPLIST
This request can be used to get the current thread list. A
pointer to an array of type lwpid_t should be passed in
addr, with the array size specified by data. The return
value from ptrace() is the count of array entries filled
in.
PT_SETSTEP This request will turn on single stepping of the specified
process.
PT_CLEARSTEP This request will turn off single stepping of the specified
process.
PT_SUSPEND This request will suspend the specified thread.
PT_RESUME This request will resume the specified thread.
PT_TO_SCE This request will trace the specified process on each
system call entry.
PT_TO_SCX This request will trace the specified process on each
system call exit.
PT_SYSCALL This request will trace the specified process on each
system call entry and exit.
PT_VM_TIMESTAMP
This request returns the generation number or timestamp of
the memory map of the traced process as the return value
from ptrace(). This provides a low-cost way for the
tracing process to determine if the VM map changed since
the last time this request was made.
PT_VM_ENTRY This request is used to iterate over the entries of the VM
map of the traced process. The addr argument specifies a
pointer to a structptrace_vm_entry, which is defined as
follows:
struct ptrace_vm_entry {
int pve_entry;
int pve_timestamp;
u_long pve_start;
u_long pve_end;
u_long pve_offset;
u_int pve_prot;
u_int pve_pathlen;
long pve_fileid;
uint32_t pve_fsid;
char *pve_path;
};
The first entry is returned by setting pve_entry to zero.
Subsequent entries are returned by leaving pve_entry
unmodified from the value returned by previous requests.
The pve_timestamp field can be used to detect changes to
the VM map while iterating over the entries. The tracing
process can then take appropriate action, such as
restarting. By setting pve_pathlen to a non-zero value on
entry, the pathname of the backing object is returned in
the buffer pointed to by pve_path, provided the entry is
backed by a vnode. The pve_pathlen field is updated with
the actual length of the pathname (including the
terminating null character). The pve_offset field is the
offset within the backing object at which the range starts.
The range is located in the VM space at pve_start and
extends up to pve_end (inclusive).
The data argument is ignored.
Additionally, machine-specific requests can exist.

RETURNVALUES

Some requests can cause ptrace() to return -1 as a non-error value; to
disambiguate, errno can be set to 0 before the call and checked
afterwards.

ERRORS

The ptrace() system call may fail if:
[ESRCH]
· No process having the specified process ID exists.
[EINVAL]
· A process attempted to use PT_ATTACH on itself.
· The request argument was not one of the legal
requests.
· The signal number (in data) to PT_CONTINUE was
neither 0 nor a legal signal number.
· PT_GETREGS, PT_SETREGS, PT_GETFPREGS,
PT_SETFPREGS, PT_GETDBREGS, or PT_SETDBREGS was
attempted on a process with no valid register set.
(This is normally true only of system processes.)
· PT_VM_ENTRY was given an invalid value for
pve_entry. This can also be caused by changes to
the VM map of the process.
[EBUSY]
· PT_ATTACH was attempted on a process that was
already being traced.
· A request attempted to manipulate a process that
was being traced by some process other than the
one making the request.
· A request (other than PT_ATTACH) specified a
process that was not stopped.
[EPERM]
· A request (other than PT_ATTACH) attempted to
manipulate a process that was not being traced at
all.
· An attempt was made to use PT_ATTACH on a process
in violation of the requirements listed under
PT_ATTACH above.
[ENOENT]
· PT_VM_ENTRY previously returned the last entry of
the memory map. No more entries exist.
[ENAMETOOLONG]
· PT_VM_ENTRY cannot return the pathname of the
backing object because the buffer is not big
enough. pve_pathlen holds the minimum buffer size
required on return.